Machine tool and its pallet changing device

ABSTRACT

The present relates to a machine tool for machining a workpiece by moving a tool and the workpiece relative to each other and to a pallet changer thereof. The machine tool is provided with a spindle for mounting the tool thereon, a table for a pallet (P) having the workpiece mounted thereon to be placed on the table, a pallet loading station for setting up the workpiece to the pallet (P) on the pallet loading station, and a pair of pallet change arms arranged to swivel about a vertical axis between the table and the pallet loading station, one pallet change arm ( 37 ) of which is arranged to be able to move independently of the other pallet change arm ( 37 ) in the vertical direction.  
     The present invention makes it possible to set the vertical position of the pallet change arm on the side of the pallet loading station to a level lower than that on the side of the table, thereby improving the efficiency and safety of the set-up job.

TECHNICAL FIELD

[0001] The present invention relates to a machine tool for machining aworkpiece by moving a tool and the workpiece relative to each other andto a pallet changer thereof, and particularly to a machine tool such asa machining center having a pallet changer attached thereto for changingpallets having a workpiece mounted thereon between a table and a palletstocker, i.e. a pallet loading station and to a pallet changer thereof.

BACKGROUND ART

[0002] Generally, a machine tool such as a machining center is providedwith a pallet changer, by which a pallet having a machined workpiecemounted thereon and placed on the table of the machine tool is exchangedwith a pallet having a set-up workpiece mounted thereon and placed on apallet loading station. Such a pallet changer is disclosed, for example,in Japanese Unexamined Utility Model Publication No. 2-53335. Thispallet changer comprises pallet change arms which are adapted to be ableto swivel about a vertical axis and to move up and down (in a verticaldirection).

[0003] The pallet changer for a machine tool disclosed in JapaneseUnexamined Utility Model Publication No. 2-53335 is configured of aframe structure including a beam arranged above the pallet change arms,an arm shaft fixed to the pallet change arms and supported verticallymovably and swingably by the beam of the frame structure, and actuatorsarranged on the beam for vertically moving the arm shaft and swivelingit. In exchanging the pallets, the pallets are released from each of thetable and the pallet loading station, and the pallet change arms arethen moved up through the arm shaft by a first actuator, thereby to liftup the pallets on the table and on the pallet loading station to anupper pallet changing position. Next, the pallet change arms areswiveled through the arm shaft by a second actuator, so that the palleton the side of the table (i.e. the machining position) is replaced withthe pallet on the side of the pallet loading station (i.e. the standbyposition). After that, the pallet change arms are moved down to a lowerpallet mounting position through the arm shaft by the first actuator,and the pallets are placed and fixed on the table and the pallet loadingstation, respectively.

[0004] In the pallet changer for the machine tool disclosed in JapaneseUnexamined Utility Model Publication No. 2-53335, the pallet change armson the sides of both the table and the pallet loading station can bemoved down only to the same vertical position along with each other. Inother words, the vertical position of the pallet on the side of thepallet loading station is set to the same vertical position as thepallet mounting position on the table.

[0005] While the workpiece on the pallet is being machined on the sideof the table, the set-up process is conducted, on the side of the palletloading station, for mounting on a pallet a next workpiece to bemachined and removing the machined workpiece from the pallet, etc. Inthe case of the pallet changer disclosed in Japanese Unexamined UtilityModel Publication No. 2-53335, however, the pallet change arms on thesides of the table and the pallet loading station can be moved down onlyto the same vertical position. Especially in a large-size machine toolsuch a machining center, this results in the very high vertical positionfor the pallet loading station, and therefore a stool or the like isrequired for the set-up job, thereby posing the problem of adeteriorated working environment and safety.

DISCLOSURE OF THE INVENTION

[0006] Accordingly, an object of the present invention is to provide amachine tool, and a pallet changer therefor, in which the verticalposition of the working surface for the pallet loading station can beset to a lower level than the upper surface of the machine tool table incarrying out the set-up job for changing the pallets between the palletloading station for set-up job and the table.

[0007] Another object of the invention is to provide a machine tool, anda pallet changer therefor, in which the vertical position of the palletchange arm on the side of the pallet loading station can be freely setindependently of the vertical position of the upper surface of thetable.

[0008] According to a first aspect of the present invention, there isprovided a machine tool for machining a workpiece by moving a tool andthe workpiece in relation to each other, which includes a spindle formounting the tool thereon, a table for a pallet having the workpiecemounted thereon to be placed on the table, a pallet loading station forconducting the set-up job thereon including the mounting/demounting ofthe workpiece on/from the pallet, and a pallet changer including a pairof pallet change arms, each pallet change arm having a pallet holder forholding selected one of a pallet on the table and a pallet on the palletloading station, the pair of the pallet change arms being swiveled abouta vertical axis thereby to exchange the pallets between the table andthe pallet loading station, the pair of the pallet change arms arrangedto be able move independently of each other in the vertical direction.

[0009] According to a second aspect of the present invention, there isprovided a machine tool for machining a workpiece by moving a tool andthe workpiece in relation to each other, which includes a spindle formounting the tool thereon, a table for a pallet having the workpiecemounted thereon to be placed on the table, a pallet loading station forconducting the set-up job including the mounting/demounting of theworkpiece on/from the pallet, and a pallet changer including a supportbody interposed between the table and the pallet loading station andable to swivel about a vertical axis, and a pair of pallet change armsmounted on the support body and extending horizontally in oppositedirections from the support body, the support body being swiveledthereby to move one of the pallet change arms from the side of the tableto the side of the pallet loading station and the other pallet changearm from the side of the pallet load station to the side of the table,the pair of the pallet change arms being able to move independently ofeach other, in the vertical direction, on the support body.

[0010] According to a third aspect of the present invention, there isprovided a machine tool for machining a workpiece by moving a tool andthe workpiece in relation to each other, which includes a spindle formounting the tool mounted thereon, a table for a pallet having theworkpiece mounted thereon to be placed on the table, a pallet loadingstation for conducting the set-up job thereon including themounting/demounting of the workpiece on/from the pallet, a base disposedbetween the table and the pallet loading station, a support body able toswivel in relation to the base about a vertical axis, swiveling meansdisposed on the base for swiveling the support body, a pair of palletchange arms, each pallet change arm including a pallet holder forholding a pallet on the table or a pallet on the pallet loading station,the pair of the pallet change arms arranged to swivel integrally withthe support body by the swiveling means and to be able to moveindependently of each other in the vertical direction on the supportbody and first vertical driving means disposed on the support body forvertically moving one of the pair of the pallet change arms, secondvertical driving means disposed on the support body in series with thefirst vertical driving means for vertically moving the pair of thepallet changer arms in cooperation with the first vertical drivingmeans, and a stopper for restricting the movement of one of the pair ofthe pallet change arms on the side of the table, when the pair of thepallet change arms are moved downward by the second vertical drivingmeans.

[0011] According to a fourth aspect of the present invention, there isprovided a machine tool for machining a workpiece by moving a tool andthe workpiece in relation to each other, which includes a spindle formounting the tool mounted thereon, a table for a pallet having theworkpiece mounted thereon to be placed on the table, a pallet loadingstation for conducting the set-up job including the mounting/demountingof the workpiece on/from the pallet, a base disposed between the tableand the pallet loading station, a support body able to swivel inrelation to the base about a vertical axis, swiveling means disposed onthe base for swiveling the support body, a pair of pallet change arms,each pallet change arm including a pallet holder for holding a pallet onthe table or a pallet on the pallet loading station, the pair of thepallet change arms arranged to swivel integrally with the support bodyby the swiveling means and to be able to move independently of eachother in the vertical direction on the support body, first verticaldriving means disposed on the support body for vertically moving one ofthe pair of the pallet change arms on the side of the pallet loadingstation, second vertical driving means disposed on the support body inseries with the first vertical driving means for vertically moving saidone of the pair of the pallet change arms on the side of the palletloading station in cooperation with the first vertical driving means,and third vertical driving means disposed on the support body inparallel to the first vertical driving means and the second verticaldriving means for vertically moving the other of the pair of the palletchange arms on the side of the table.

[0012] According to a fifth aspect of the present invention, there isprovided a pallet changer which includes a support body able to swivelabout a vertical axis and a pair of pallet change arms extendinghorizontally in opposite directions from the support body, the supportbody being swiveled thereby to move one of the pair of the pallet changearms from a machining position to a standby position and the otherpallet change arm from the standby position to the machining position,wherein the pair of the pallet change arms are arranged to able to moveindependently of each other in the vertical direction on the supportbody.

[0013] In the pallet changer described above, preferably, during thedownward movement of the pair of the pallet change arms, one of the pairof the pallet change arms can further move downward after the otherpallet change arm stops. More preferably, the pair of the pallet changearms are moved in the vertical direction by a single operating roddriven by two cylinder units connected in series to each other, so thatthe pair of the pallet change arms can assume three different verticalpositions, and that a stopper restricts the downward movement of one ofthe pair of the pallet change arms. Further, each of the pair of thepallet change arms may be moved in the vertical direction by a differentoperating rod driven by a different cylinder unit.

[0014] The configuration in which a pair of the pallet change arms canmove independently of each other in the vertical direction makes itpossible to move each of the pair of the pallet change arms in adifferent vertical position (height). Thus, one pallet (on the side ofthe pallet loading station) is not required to be set to the same heightas the other pallet (on the side of the table), thereby to allow onepallet (on the side of the table) to be set freely to any height.

[0015] Especially, in view of the fact that one of the pair of thepallet change arms can move downward further than the other palletchange arm, the standby position (on the side of the pallet loadingstation) usable for the setup job, etc. can be set lower than the palletin the machining position (on the side of the table) facing the machinetool.

[0016] Further, the downward movement of one of the pair of the palletchange arms driven by the vertical driving means is restricted by astopper, so that the other pallet change arm alone can move downfurther.

[0017] Also, by moving the two pallet change arms independently of eachother by different vertical driving means, only one of the pallet changearms can be moved downward further than the other pallet change arm.

[0018] With the configurations and operations described above, accordingto the present invention, a pair of pallet change arms are movableindependently of each other in the vertical direction, and therefore thevertical position of the pallet change arm on the side of the palletloading station can be set to a level lower than the vertical positionof the pallet change arm on the side of the table of the machine tool.Thus, the working surface of the pallet loading station can be set low,thereby improving the efficiency and safety of the set-up job.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other objects, features and advantages of thepresent invention will be described in more detail below with referenceto embodiments according to the present invention and in conjunctionwith the accompanying drawings.

[0020]FIG. 1 is a perspective view showing a pallet changer of a machinetool according to the present invention.

[0021]FIG. 2 is a partial sectional side view showing the internalstructure of the pallet changer of the machine tool shown in FIG. 1.

[0022]FIG. 3 is a plan view of a machine tool provided with the palletchanger shown in FIGS. 1 and 2.

[0023]FIG. 4A is a schematic diagram showing the operation of a palletchanger of a machine tool according to the present invention, whenmounting a pallet on the table of the machine tool. rig. 4B is aschematic diagram showing the operation of the pallet changer for themachine tool according to the present invention, when exchanging thepallet on the side of the table with the pallet on the side of thepallet loading station.

[0024]FIG. 4C is a schematic diagram showing the operation of the palletchanger for the machine tool according to the present invention, whenmounting the pallet on the pallet loading station.

[0025]FIG. 5 is a schematic diagram showing a pallet changer for amachine tool according to another embodiment of the present invention.

[0026]FIG. 6 is a schematic diagram showing a pallet changer for amachine tool according to still another embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0027] First, a machine tool according to an embodiment of the presentinvention will be described with reference to FIG. 3.

[0028] A machine tool 11 includes a bed 13, a column 15, a spindle head17 mounted on the column 15, a spindle 19, a table 21 and a palletloading station 23 for changing a pallet. In order that a pallet Pplaced on the table 21 and having a machined workpiece (not shown)mounted thereon may be replaced with another pallet P placed on thepallet loading station 23 and having a set-up workpiece (not shown)mounted thereon, a pallet changer 25 is disposed between the table 21and the pallet loading station 23 of the machine tool 11.

[0029] In the machine tool 11 shown in FIG. 3, the spindle 19 isarranged horizontally, and the spindle head 17 is mounted to be able tomove in the vertical direction (in a direction of a Y-axis) on thecolumn by a well-known slide guide mechanism. The column 15 is movablein the lateral direction (in a direction of an x-axis) on the bed 13, asviewed from the front of the machine tool 11, while the table 21 ismovable in the backward and forward direction (in a direction of aZ-axis) on the bed 13. Thus, the spindle 19 is movable in relation tothe table 21 in the three directions along the X-, Y- and Z-axes.Further, a tool magazine 27 for an automatic tool changer is disposed onthe left side of the spindle 19, and a NC unit 29 is disposed on theright side of the spindle 19.

[0030] Four tapered cones (not shown) are disposed on the upper surfaceof the table 21, and the pallet 7 having the workpiece mounted thereonare formed with corresponding depressions. Therefore, the pallet P canbe accurately supported in position simply by placing it on the table21. AS described above, the table 21 is movable in the backward andforward direction (along the Z-axis), and therefore is adapted to bemovable in relation to the spindle 19 for the machining operation,thereby allowing the table 21 to move to the position of the pallet Pindicated by chain line at the time of changing the pallet.

[0031] Next, the pallet changer 25 of the machine tool 11 according tothe present invention shown in FIG. 1 will be described in detail withreference to FIGS. 1 and 2.

[0032] Referring to FIG. 1, the pallet changer 25 includes a base 31arranged on the bed 13 of the machine tool 11, an arm base, i.e. asupport body 35 extending vertically from the base 31 and able to swivelabout the vertical axis 33, and a pair of pallet change arms 37extending horizontally in opposite directions from the support body 35.Normally, one of the two pallet change arms 37 is arranged at a positionnearer to the machine tool 11 called “the machining position”, while theother pallet change arm 37 is arranged at a position nearer to thepallet loading station 23 called “the standby position”.

[0033] Each of pallet change arms 37 includes a slide plate 37 a movablein the vertical direction on the support body 35 along a pair of guides39 located on the support body 35, and a forked pallet holder 37 bextending in the horizontal direction from the slide plate 37 a, withthe forward end thereof branching into two parts. The pallet change arm37 is mounted movably on the support body 35 by the slide plate 37 a.The upper portion of the slide plate 37 a further includes a projectedportion 37 c projecting from the slide plate 37 a and extending in thehorizontal direction.

[0034] A pair of the pallet change arms 37 are driven in the verticaldirection by the vertical driving means extending through the supportbody 35. Referring to FIG. 2, the vertical driving means includes afirst cylinder unit (i.e. first vertical driving means) 41 mounted onthe base 31 and extensible in the vertical direction, and a secondcylinder unit (i.e. second vertical driving means) 47 connected inseries with the first cylinder unit 41 though the operating end portion45 of the operating rod 43 of the first cylinder unit 41 and extensiblein the vertical direction. The operating end portion 51 of the operatingrod 49 of the second cylinder unit 47 and the projected portion 37 c ofthe pallet change arm 37 come into contact with each other, so that thedriving force of the vertical driving means is transmitted to the palletchange arm 37, and the pallet change arm 37 moves vertically on thesupport body 35 along the guides 39.

[0035] In the embodiment shown in FIG. 2, the support body 35 is furthercoupled to the operating end portion 45 of the first cylinder unit 41,so that, in accordance with the extension or contraction of the firstcylinder unit 41 in the vertical direction, the support body 35 is movedin the vertical direction together with the second cylinder unit 47.

[0036] The support body 35 can swivel in relation to the base 31 aboutthe vertical axis 33 by the swiveling force applied thereto from theswiveling means. The swiveling means may be configured of, for example,a rack-pinion mechanism as shown in FIG. 2.

[0037] Referring to FIG. 2, the pinion 53 of the swiveling means is of acylindrical shape with the vertical axis 33 as a rotational axis, andarranged as to surround the first cylinder unit 41. In FIG. 2, the firstcylinder unit 41 surrounded by the cylindrical pinion 53 is shown bydotted line to facilitate the description. one end of the cylindricalpinion 53 is further connected fixedly to the bottom portion of thesupport body 35. The rack 55 in mesh with the pinion 53 is driven in thehorizontal direction by an appropriate driving unit (not shown), so thatthe support body 35 swivels about the vertical axis 33, thereby causingthe pair of pallet change arms 37 to swivel about the vertical axis 33.

[0038] The support body 35 to which the pinion 53 is connected iscoupled with the first cylinder unit 41. In accordance with theextension/contraction of the first cylinder unit 41, therefore, thesupport body 35 moves in relation to the base 31 in the verticaldirection, so that the pinion 53 connected to the support body 35 alsomoves in relation to the base 31 in the vertical direction. In orderthat the rack 55 arranged on the base 31 may be kept in engagement withthe pinion 53 even when the pinion 53 moves in relation to the base 31in the vertical direction, the pinion 53 is elongated along the verticaldirection.

[0039] Although the embodiment shown in FIG. 2 employs the rack-pinionmechanism for swiveling the support body 35, other appropriate swivelingmechanisms can of course be used alternatively.

[0040] As described above, the two cylinder units 41, 47 are connectedin series with each other, so that the pair of the pallet change arms 37can be driven in the vertical direction by one operating rod 49 drivenby the cylinder units 41, 47. This allows the pallet change arms 37 toassume three different positions (heights) in the vertical direction.

[0041] In the case where the pair of the pallet change arms 37 are movedby the first and second cylinder units 41, 47 connected in series witheach other, however, the two pallet change arms 37 can assume only thesame vertical position. in view of this, in the embodiment shown in FIG.2, a stopper 57 is disposed on one side of the base 31 in order torestrict the movement (i.e. downward movement) of one (normally, thepallet change arm 37 in standby position) of the pair of the palletchange arms 37 during the downward movement of the pair of the palletchange arms 37.

[0042] As the result of provision of this stopper 57, while the pair ofthe pallet change arms 37 are being moved by a first and second cylinderunits 41, 47 connected in series with each other, one (the pallet changearm in the standby position) of the pair of the pallet change arms 37can be further moved downward after the other pallet change arm (in themachining position) 37 stops during the downward movement. In the palletchanger 25, therefore, the set-up job can be carried out on the side ofthe pallet change arm 37 capable of moving to a lower vertical position,while the pallet change arm 37 with the vertical position thereofrestricted by the stopper 57 during the downward movement can be causedto face the machine tool. By doing this, the vertical position of thepallet change arm 37 in the standby position is not limited by theheight of the table 21 of the machine tool 11.

[0043] Next, the operation of the pallet changer 25 of the machine toolshown in FIGS. 1 and 2 will be described with reference to FIGS. 4A to4C. In FIGS. 4A to 4C, the support body 35, etc. are omitted for thepurpose of simplification.

[0044] While the pallet P is placed on the table 21 of the machine tool11 and the workpiece mounted on the pallet P is being machined, the twopallet change arms 37 of the pallet changer 25 of the machine tool 11according to the present invention stand by at the vertical positionshown in FIG. 4A (where the pallet is placed and mounted on the table21, hereinafter referred to as the table mounting position). When themachining of the workpiece is completed, the table 21 moves toward thepallet changer 25, so that the pallet P having the machined workpiecemounted thereon is transferred to the pallet change arm 37 of the palletchanger 25. Then, the first cylinder unit 41 extends to move the twopallet change arms 37 up to the vertical position shown in FIG. 4B(hereinafter referred to as the pallet changing position). As a resultof the movement of the pallet change arms 37 up to the pallet changeposition, the base 31 ceases to prevent the swivel of the support body35 and the pallet change arms 37. Thus, the support body 35 is swiveledby the rack-and-pinion mechanism, so that one pallet change arm 37 isswiveled from the machining position to the standby position while theother pallet change arm 37 is swiveled from the standby position to themachining position.

[0045] Next, the first cylinder unit 41 is contracted thereby to movethe two pallet change arms 37 down to a vertical position where thepallet change arm 37 in the machining position (i.e. on the side of themachine tool) comes into contact with the stopper 57. At this time, thetwo pallet change arms 37 will assume the table mounting position, andthe pallet P held by the pallet change arm 37 in the machining positionwill be mounted on the table 21 of the machine tool 11.

[0046] Further, after one (the pallet change arm in the machiningposition) of the pair of the pallet change arms 37 is stopped by thestopper 57, the second cylinder unit 47 is contracted to move the otherpallet change arm 37 down to the vertical position shown in FIG. 4c(where the pallet is placed and mounted on the pallet loading station,hereinafter referred to as the pallet loading station mountingposition). Under this condition, the pallet change arm 37 in themachining position is stopped by the stopper 57, and therefore cannotmove down below the table mounting position. Thus, only the palletchange arm 37 in the standby position moves down to the pallet loadingstation mounting position, with the result that the pallet P held in thepallet change arm 37 in the standby position is mounted on the palletloading station 23.

[0047] In this way, the pair of the pallet change arms 37 can be movedin the vertical direction independently of each other on the supportbody 35 to assume different vertical positions respectively. As aresult, the vertical position of the pallet P in the standby positioncan be set lower than that of the pallet P in the machining position, sothat the set-up job on the pallet loading station 23 is facilitated foran improved efficiency and safety of the set-up job.

[0048] When the set-up job for the workpiece is completed on the palletloading station 23, on the side of which the pallet change arm 37 in thestandby position is arranged, the second cylinder unit 47 is extended tomove the pallet change arm 37 up to the table mounting position, asshown in FIG. 4A. Under the condition shown in FIG. 4C, only theprojected portion 37 c of the pallet change arm 37 in the standbyposition is in contact with the operating end portion 51 of theoperating rod 49, and therefore only the pallet change arm 37 in thestandby position moves up.

[0049] By moving the pallet change arms 37 in the standby position up tothe table mounting position as shown in FIG. 4A when the set-up job iscompleted in the manner described above, it can be avoided that the timerequired for moving the pallet change arm 37 in the standby positionbetween the pallet loading station mounting position and the tablemounting position otherwise might have an effect on the machining time.Consequently, in spite of the capability of moving the pallet change arm37 in the standby position down to a position lower than the palletchange arm 37 in the machining position, the machining time with thepallet changer 25 of the machine tool 11 according to the presentinvention is not longer than that with the conventional pallet changerin which a pair of the pallet change arms 37 cannot assume other thanthe same vertical position.

[0050] Although the pallet changer 25 of the machine tool 11 accordingto the present invention is described above with reference to anembodiment in which the pallet change arms 37 are moved in the verticaldirection by the vertical driving means including the first and secondcylinder units 41, 47 connected in series to each other, it is notlimited to the embodiment described above. Also, since the scope of thepresent invention is defined by the appended claims, any modificationsand changes can be made without departing from the scope of the sameclaims.

[0051] As shown in FIG. 5, for example, the first and second cylinderunits 41, 47 connected in series to each other may be suspended from theceiling thereby to move the two pallet change arms 37 with a singleoperating rod 49 driven by the first and second cylinder units 41, 47connected in series to each other. in such a case, as in the embodimentshown in FIGS. 1 and 2, the stopper 57 is required for restricting thedownward movement of only one pallet change arm 37.

[0052] Also, as shown in FIG. 6, each of the two pallet change arms 37may be driven by an independent vertical driving means. In theembodiment shown in FIG. 6, one of the pallet change arms 37 is drivenby the two cylinder units 59 connected in series to each other, whilethe other pallet change arm 37 is driven by one cylinder unit 61.Therefore, the former pallet change arm 37 can take three differentvertical positions, while the latter pallet change arm 37 can take twodifferent vertical positions. Thus, the same operation as in theembodiment shown in FIGS. 1 and 2 can be performed without any stopper.

[0053] Further, in the embodiment shown in FIGS. 1 and 2, a singlecylinder unit capable of taking three or more different positions can beemployed in place of the first and second cylinder units 41, 47connected in series to each other.

[0054] Incidentally, the support body 35, etc. are omitted in FIGS. 5and 6 as in FIGS. 4A to 4C for the purpose of simplification.

1. A machine tool for machining a workpiece by moving a tool and theworkpiece in relation to each other, said machine tool comprising: aspindle for mounting said tool thereon; a table for a pallet having saidworkpiece mounted thereon to be placed on said table; a pallet loadingstation for conducting the set-up job thereon including themounting/demounting of said workpiece on/from said pallet; and a palletchanger including a pair of pallet change arms, each pallet change armhaving a pallet holder for holding selected one of a pallet on saidtable and a pallet on said pallet loading station, said pair of thepallet change arms being swiveled about a vertical axis thereby toexchange the pallets between said table and said pallet loading station,said pair of the pallet change arms arranged to be able to moveindependently of each other in the vertical direction.
 2. A machine toolfor machining a workpiece by moving a tool and the workpiece in relationto each other, said machine tool comprising: a spindle for mounting saidtool thereon; a table for a pallet having said workpiece mounted thereonto be placed on said table; a pallet loading station for conducting theset-up job thereon including the mounting/demounting of said workpieceon/from said pallet; and a pallet changer including a support bodyinterposed between said table and said pallet loading station and ableto swivel about a vertical axis, and a pair of pallet change armsmounted on said support body and extending horizontally in oppositedirections from said support body, said support body being swiveledthereby to move one of said pallet change arms from the side of saidtable to the side of said pallet loading station and the other palletchange arm from the side of said pallet load station to the side of saidtable, said pair of the pallet change arms being able to moveindependently of each other in the vertical direction on said supportbody.
 3. A machine tool for machining a workpiece by moving a tool andthe workpiece in relation to each other, said machine tool comprising: aspindle for mounting said tool thereon; a table for a pallet having theworkpiece mounted thereon to be placed on said table; a pallet loadingstation for conducting the set-up job thereon including themounting/demounting of said workpiece on/from said pallet; a basedisposed between said table and said pallet loading station, a supportbody able to swivel in relation to said base about a vertical axis;swiveling means disposed on said base for swiveling said support body; apair of pallet change arms, each pallet change arm including a palletholder for holding a pallet on said table or a pallet on said palletloading station, said pair of the pallet change arms arranged to swivelintegrally with said support body by said swiveling means to be able tomove independently of each other in the vertical direction on saidsupport body; first vertical driving means disposed on said support bodyfor vertically moving one of said pair of the pallet change arms; secondvertical driving means disposed on said support body in series with saidfirst vertical driving means for vertically moving said pair of thepallet change arms in cooperation with said first vertical drivingmeans; and a stopper for restricting the movement of one of the pair ofpallet change arms on the side of said table, when said pair of thepallet change arms are moved downward by said second vertical drivingmeans.
 4. A machine tool for machining a workpiece by moving a tool andthe workpiece in relation to each other, comprising: a spindle formounting said tool thereon; a table for a pallet having said workpiecemounted thereon to be placed on said table; a pallet loading station forconducting the set-up job including the mounting/demounting of saidworkpiece on/from said pallet; a base disposed between said table andsaid pallet loading station; a support body able to swivel in relationto said base about a vertical axis; swiveling means disposed on saidbase for swiveling said support body; a pair of pallet change arms, eachpallet change arm including a pallet holder for holding a pallet on saidtable and a pallet on said pallet loading station, said pair of thepallet change arms arranged to swivel integrally with said support bodyby said swiveling means and to be able to move independently of eachother in the vertical direction on said support body; first verticaldriving means disposed on said support body for vertically moving one ofsaid pair of the pallet change arms on the side of said pallet loadingstation; second vertical driving means disposed on said support body inseries with said first vertical driving means for vertically moving saidone of said pair of the pallet change arms on the side of said palletloading station in-cooperation with said first vertical drive means; andthird vertical driving means disposed on said support body in parallelto said first vertical driving means and said second vertical drivingmeans for vertically moving the other of said pair of the pallet changearms on the side of said table.
 5. A pallet changer comprising a supportbody able to swivel about a vertical axis and a pair of pallet changearms extending horizontally in opposite directions from said supportbody, said support body being swiveled thereby to move one of said pairof the pallet change arms from a machining position to a standbyposition and the other pallet change arm from the standby position tothe machining position; wherein said pair of the pallet change arms arearranged to be able to move independently of each other in the verticaldirection on said support body.
 6. The pallet changer according to claim5, wherein during the downward movement of said pair of the palletchange arms, one of said pair of the pallet change arms can further movedownward after the other pallet change arm stops.
 7. The pallet changeraccording to claim 6, wherein said pair of the pallet change arms aremoved in the vertical direction by a single operating rod driven by twocylinder units connected in series to each other, so that said pair ofthe pallet change arms can assume three different vertical positions,and wherein a stopper restricts the downward movement of one of saidpair of the pallet change arms.
 8. The pallet changer according to claim5, wherein each of said pair of the pallet change arms is moved in thevertical direction by a different operating rod driven by a differentcylinder unit.